Admittedly, I haven't seen the ballistics test of the 7.5 inch 5.56 so I can't really speak to that although 50 yards is certainly plausible for most 5.56 bonded ammo. I am only privy to the testing on the 10.5 and up.
Robo has an excellent point that there aren't a lot of Patrol Rifle engagements that will exceed 50 yards (likely single digit incidents nationwide each year) and your wife will likely be ok no matter which length she goes with. I mean seriously the MK18 has a good track record of solving problems. A short rifle with a reduced velocity bullet is preferable over a handgun.

In what regards? It was touched on earlier, but she needs enough barrel length to keep the velocity at a point so the bullet can do its job. Looking cool do not end critical situations. SRT's mission for the department isn't the same as patrol. Just like bigger isn't always better in the bedroom, shorty isn't always better in a tactical environment. Over the years, I have been issued a variety of SBR's from 10.5 -14.5 and aesthetics aside, my favorites are 11.5 and 14.5. I have an 11.5 with micro RDS and 14.5 with ACOG/RMR. The 11.5 only has an edge in vehicle operations (in my opinion). I run them interchangeably during CQB. Although I don't know which of the CQB methodologies that particular SRT adheres to, I can say the types of missions that would be hampered (slowed) by those 3 extra inches fall into a VERY narrow category.
The 12.5 is a legit compromise for a patrol rifle if you can find a manufacture that suits your needs.
A properly built 11.5 -14.5 upper with quality components will "hang" with the SRT guys.

Bam! No sense in killing the life of a expensive barrel on 15 yards and in drills. The Brownells B-Tac barrel is my go to barrel for these situations. It is tough as hell. I don't know who make them for Brownells, but they are a solid purchase for abusive shooting.

I keep notes (shooter's log) on my precision range days. Mainly because I have found that I am the weakest link in the system and I don't like making major changes to a system until I know it is the system. We all have days that our focus is off or we just can't hone in our trigger work or whatever. I would take it out a few more times on different days, same lot of ammo, same type target, then lets see where you are with the POI shifts.

Cold Bore
Cold Fouled Bore
Cold Clean Bore
Cold Shooter
I have heard a lot of arguments through the years and none of them has scientifically swayed me either way. There are people that are pretty passionate about their position. I have tracked one or two cold bore shots throughout the years and FOR ME...my conclusion is that Cold Shooter and Cold Clean Bore contribute to the largest POI shifts. Cold Shooter and Cold Fouled Bore have less POI shifts. These shifts are around .1 or at the most .2 mil (ETA: maybe .3) shifts at 100 yards. Nothing like .5 or .6 Mils. If .5 Mils or more is your cold bore shift, you have more going on.

I only see the SOCC Battlerail on sale. I don't have experience with them, but I do have a lot of experience with the Alpha and Bravo rails and they are solid rails. I currently run the Geissele M4 and they are certainly a step up.